JP2015056263A - Separator for batteries - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a separator for batteries excelling in internal resistance.SOLUTION: In a separator for batteries combining a porous ceramic layer containing ceramic particulates and a supporting medium (porous film, woven cloth, unwoven cloth knit cloth or the like), the ceramic particulates are irregular-shaped silica (long-shaped silica, such as rod-shaped, bent or branched one, obtained by adding a salt forming a basic oxide to active silicic acid and performing hydrothermal synthesis, which serves as the raw material silica).

Description

  The present invention relates to a battery separator.

  Conventionally, as a separator of a lithium secondary battery, a polyolefin porous film having fine pores that have penetrated has been used. These separators suppress heat generation by increasing the internal resistance of the battery when the battery malfunctions and generate heat, and the through-holes are melted and closed. It has been responsible for suppressing battery explosion due to runaway.

  However, in applications that require charging and discharging with a large current, such as batteries for hybrid vehicles and uninterruptible power supplies, research on electrode composition has made it possible to suppress thermal runaway explosions, and conversely, rapid battery internals Due to the increase in temperature, in order to avoid electrode contact due to thermal contraction of the separator, there is an increasing demand for a separator having high heat resistance and low internal resistance.

  In response to this demand, Patent Document 1 discloses that the battery is thermally runaway by combining a porous ceramic layer containing porous ceramic fine particles with a porous support such as a nonwoven fabric. There has been proposed a method capable of suppressing electrode contact without causing thermal contraction of the separator even in the case of occurrence of heat. This method is an excellent method because the porous ceramic layer can permeate the surface and the inside of the separator and can impart high electrolyte retention and heat resistance.

  On the other hand, Patent Documents 2 and 3 propose a method of providing heat resistance by providing a porous ceramic layer containing ceramic fine particles on one side of a porous film. In particular, Patent Document 2 describes that titanium oxides such as alumina, zirconia, silica, titania, barium titanate, lead titanate, strontium titanate, and composite oxides thereof are effective for the porous ceramic layer.

  In particular, silica having a high insulating property is an inexpensive and very promising material, but if it is simply granular, the internal resistance cannot be lowered, leaving a problem.

Japanese Patent No. 4594098 Special table 2008-503049 gazette Japanese Patent No. 4499851

  An object of the present invention is to provide a battery separator having excellent internal resistance.

  As a result of intensive studies, the present inventors have found that the above-described problems can be solved by the present invention described below.

[1] A battery separator in which a porous ceramic layer containing ceramic fine particles and a support are combined, wherein the ceramic fine particles are deformed silica.

  In the present invention, a battery separator having excellent internal resistance can be obtained.

  The battery separator of the present invention is a battery separator in which a porous ceramic layer containing ceramic fine particles and a support are combined, wherein the ceramic fine particles are deformed silica.

  Most of wet silica using dry silica, water glass or organic silicate compounds as a raw material by a gas phase method is a granular material. The coated film of silica particles is formed by a close-packed structure of particles and can be used as an inorganic binder, but the porosity of the close-packed structure of spherical particles is theoretically 26% of the space capacity and contains granular silica. In the porous ceramic layer formed as described above, the ability to retain the electrolytic solution is lowered, so that it is difficult to realize a separator having a low internal resistance. In the present invention, deformed silica obtained by hydrothermal synthesis is contained in the porous ceramic layer.

  The irregular-shaped silica is a rod-like, bent, or branched elongated silica obtained by hydrothermal synthesis by adding a salt that forms a basic oxide to active silicic acid that is a raw material silica. The active silicic acid is a silica sol obtained by subjecting water glass to a cation exchange treatment and adjusting the pH to an acidic atmosphere. As such salts, calcium, magnesium chloride, nitrate, sulfamate, formate, acetate and the like are preferable materials. The addition amount is about 0.15 mass% to 1.5 mass% in terms of CaO and MgO (basic oxide), and the pH is returned from neutral to alkaline to about 85 to 200 ° C. Heat for about 5 to 20 hours to obtain deformed silica having a predetermined structure. During hydrothermal synthesis, if the silica concentration is too high, the silica concentration is preferably 10 to 40% by mass due to problems such as the gelation of the silica sol or the sudden bond bonding that makes the reaction uncontrollable. More preferably, it is 15-30 mass%. Such deformed silica is, for example, ST-UP, ST-UP-S, ST-UP-M, ST, which are specially shaped products, manufactured by Nissan Chemical Industries, Ltd. under the trade name Snowtex (registered trademark). -Available in grades such as OUP, ST-UP-SO, ST-UP-MO.

In the present invention, the surface of the irregular shaped silica can be treated with an alkali. The surface hydroxyl group of silica is on the acidic side, and protons are easily dissociated in water. In the battery, it is used after sufficiently removing the water adsorbed on the surface of the silica, but a trace amount of water remains and the surface hydroxyl group cannot be completely removed. When exposed to the electrolyte in this state, surface hydroxyl groups release protons and adsorb lithium ions. Protons generated by the adsorption of lithium ions react with anions such as PF ₆ ⁻ which are electrolyte counter ions, and in this case, HF (hydrogen fluoride) and PF ₅ are generated. Further, hydrogen fluoride attacks the electrode agent and degrades battery characteristics. In particular, this reaction is promoted in an environment where the battery is heated, and the generated hydrogen fluoride easily breaks the silicon-oxygen bond of silica, and the fluorine ion directly binds to silicon. The reaction is chained and the separator itself is deteriorated. In order to suppress this reaction, it is necessary to suppress the generation of dissociated protons.

  As a method of suppressing dissociated protons on the silica surface, a method of treating the surface of the irregular silica with an alkali and blocking the surface hydroxyl group of the silica with an alkali is simple and effective. Examples of alkalis include ammonia, nitrogen-containing compounds, and basic metal salts. In particular, sodium hydroxide, potassium hydroxide, lithium hydroxide, cesium hydroxide, barium hydroxide, strontium hydroxide, water Hydroxides such as calcium oxide and magnesium hydroxide are preferred. In either case, in the electrolyte solution containing the lithium-based supporting electrolyte, generation of free protons can be suppressed, and generation of hydrogen fluoride that causes a change in the chemical structure of the battery can be suppressed.

  In the presence of an alkali such as sodium hydroxide, the deformed silica surface in water dissociates protons and is negatively charged, and this charge stably disperses in water. Next, when taken out from water to form a porous film, sodium ions are adsorbed on the surface to form a blocking layer. A similar phenomenon occurs with other alkalis. The alkali coating amount is preferably 0.01% by mass to 10% by mass of silica, and more preferably 0.02% by mass to 2% by mass. Modified silica treated with sodium hydroxide is available as ST-UP, ST-UP-S, ST-UP-M in the trade name Snowtex (registered trademark) manufactured by Nissan Chemical Industries.

  In the present invention, the porous ceramic layer contains irregular shaped silica. The deformed silica is kept at the maximum average distance by mutual charge until water is removed, and loses water and aggregates in the nearest part. This aggregated structure has a structure that repels each other, and most ideally forms a longer rod-like structure. Of course, it is conceivable that the surface tension of water overcomes this during the drying process, or that the surface-active agent supplements the structure and the rod-like silica aggregates in parallel, but in any case, it is different from the spherical silica. The internal porosity can be increased by self-forming a highly anisotropic aggregate structure.

  In the present invention, the porous ceramic layer can contain a water-soluble cellulose derivative. The ceramic fine particles are dispersed in water to form a coating liquid that forms a porous ceramic layer. In this case, if a water-soluble cellulose derivative is contained, the water-soluble cellulose derivative becomes a dispersibility aid in water, and the coating liquid is thickened to improve the setting property of the coating liquid on the support. be able to. As the dispersing agent at the time of dispersion, various surfactants such as nonionic, anionic, and cationic, and salts for imparting electric charge to the ceramic fine particles can be used in combination.

  The water-soluble cellulose derivative in the present invention is a cellulose derivative synthesized by modifying a part of hydroxyl groups of β-glucose molecules bonded linearly by glycosidic bonds so as to be water-soluble. , A compound modified with a carboxymethoxy group, a methoxy group, a hydroxyethoxy group, or a hydroxyproxy group. A derivative substituted with a carboxymethoxy group is called carboxymethylcellulose (CMC), and can be water-solubilized with sodium salt or ammonium salt. Methylcellulose containing only methoxy groups dissolves only in low-temperature water and has a thermal gel property that gels an aqueous solution when the temperature rises. Moreover, it is excellent in foaming property and foaming property, and can behave like a nonionic polymer surfactant. In general, solubility and thermal gel properties can be controlled by combining a hydroxyethoxy group or a hydroxypropoxy group with a methoxy group. In addition, water-soluble cationized cellulose can also be used. However, cellulose derivatives such as cellulose acetate and ethyl cellulose cannot be used because they are water-insoluble cellulose derivatives that do not dissolve in water. The water-soluble cellulose derivative can be used in combination with ceramic fine particles to form a porous ceramic layer and reduce internal resistance. If the content of the water-soluble cellulose derivative is too large, a film is formed around the voids in the drying step and independent voids are formed. Therefore, the content is preferably 5% by mass or less of the porous ceramic layer, more preferably 3 It is below mass%.

  Various polymer binders can be used in combination in order to improve the adhesion between the ceramic particles and the adhesion between the support and the ceramic particles. In particular, when polyester or polypropylene, which is difficult to bond, is used for the support, it is preferable to use a latex polymer binder as the polymer binder. As the polymer binder, polyolefin, styrene-butadiene, acrylic, or the like can be used. The content of the polymer binder is preferably 0.5 to 20% by mass of the porous ceramic layer, and more preferably 1 to 8% by mass. Furthermore, various surfactants such as sodium laurate and dodecylbenzenesulfonic acid can be used in combination in order to improve the dispersibility of the particles and the affinity with the support.

  In the present invention, a porous ceramic layer is combined with a support in order to improve the strength as a battery separator. Examples of the support include a porous film, a woven fabric, a nonwoven fabric, and a knitted fabric. Examples of the material for the support include polyester, polyolefin, polyamide, aramid, and cellulose. The support is a non-woven fabric using fibers such as polyester, polyolefin, polyamide, aramid, cellulose, etc., and is particularly excellent in heat resistance and easy to obtain fine fibers, using non-woven fabric using polyester or polyolefin fibers. Is preferred. The nonwoven fabric can be produced by various methods such as a wet method, a dry method, and an electrostatic spinning method. As a support body, it is preferable that it is 10-25 micrometers in thickness, and it is preferable that a porosity is 30-80%. More preferably, the thickness is 12 to 18 μm and the porosity is 40 to 70%.

  When wet-made non-woven fabric is used as the support, the surface of the support is hydrophilized by the surfactant remaining on the fiber surface at the time of paper-making, but remains in large quantities and is redissolved in the electrolyte. Therefore, it is preferable that the surface of the support is pretreated on the surface on which the deformed silica can be set. Such a surface is preferably a crosslinked film made of a polymer capable of imparting hydrophilicity, such as hydrophilic polyester, polyvinyl alcohol, polyvinyl acrylamide, gelatin or chitosan. In addition, fibrous boehmite that adsorbs on the fiber surface and self-organizes is also a preferable material.

The porous ceramic layer can be obtained by applying or casting a coating liquid containing ceramic fine particles on a support, gelling in some cases, and then drying. As an application or casting method, an air doctor coater, blade coater, knife coater, rod coater, squeeze coater, impregnation coater, gravure coater, kiss roll coater, die coater, reverse roll coater, transfer roll coater, spray coater, etc. The method used can be used. The coating amount of the porous ceramic layer is preferably 0.5 to 50 g / ^{m 2} by dry weight, more preferably from 1 to 30 g / ^{m 2.} It is also possible to adjust the thickness of the obtained battery separator by performing another heat calendering process after drying. The preferable thickness of the battery separator having a support is 10 to 30 μm, more preferably 12 to 25 μm.

  The obtained battery separator is cut and sandwiched between electrode materials for a lithium secondary battery, an electrolyte is injected, the battery is sealed, and a lithium secondary battery is obtained. The material constituting the positive electrode is mainly an active material and a conductive agent such as carbon black, a binder such as polyvinylidene fluoride and styrene butadiene rubber, and as the active material, lithium cobaltate, lithium nickelate, lithium manganate, A lithium manganese composite oxide such as lithium nickel manganese cobaltate (NMC) or lithium aluminum manganate (AMO), lithium iron phosphate, or the like is used. These are mixed and applied onto an aluminum foil as a current collector to form a positive electrode.

The material constituting the negative electrode is mainly an active material, a conductive agent, and a binder. As the active material, graphite, amorphous carbon material, silicon, lithium, lithium alloy and the like are used. These are mixed and applied onto a copper foil as a current collector to form a negative electrode. In a lithium secondary battery, a separator is sandwiched between a positive electrode and a negative electrode, and an electrolytic solution is impregnated therein to provide ionic conductivity and conduct. In the lithium secondary battery, a non-aqueous electrolyte solution is used. Generally, this is composed of a solvent and a supporting electrolyte. As the solvent, for example, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and vinylene carbonate having an additive function, vinyl Carbonate type such as ethylene carbonate. Dimethoxyethane (DME) can also be used. As the supporting electrolyte, in addition to lithium hexafluorophosphate and lithium tetrafluoroborate, an organic lithium salt such as LiN (SO ₂ CF ₃ ) ₂ is also used. Ionic liquids can also be used. Further, a gel electrolyte in which a lithium salt is dissolved in a gel polymer such as polyethylene glycol or a derivative thereof, polymethacrylic acid derivative, polysiloxane or a derivative thereof, or polyvinylidene fluoride can also be used.

  As the exterior body, a metal cylindrical can such as aluminum or stainless steel, a rectangular can, a sheet-type exterior body using a laminate film obtained by laminating aluminum foil with polypropylene, polyethylene, polyethylene terephthalate, polybutylene terephthalate, or the like can be used. Further, it can be used by stacking and stacking, or it can be used by rotating in a cylindrical shape.

  EXAMPLES Next, although an Example demonstrates this invention further in detail, this invention is not limited to these at all.

[Production of support]
With a constitution of 75 parts by mass of stretched regular polyethylene terephthalate (PET) fibers (0.1 dtex, length 3 mm) and 25 parts by mass of unstretched PET fibers (0.2 dtex, length 4 mm), the weight per unit area is 8.5 g / An m ² web was prepared. The drying temperature at this time was 130 ° C. Next, a thermal calendar process was performed on the web at 220 ° C. to prepare a support that was a wet nonwoven fabric having a thickness of 15 μm.

Next, as a cationic polymer, chitosan (manufactured by Koyo Chemical Co., Ltd., trade name: Koyo Chitosan SK-400) and a vinyl sulfone compound (manufactured by Fuji Film, special cross-linking agent VS-B) were adjusted to 100/3 by mass part ratio. The mixed coating liquid was impregnated so as to have a dry mass of 0.2 g / m ² to coat the fiber, and this was used as a base material.

Example 1
Deformed silica (manufactured by Nissan Chemical Industries, trade name: Snowtex (registered trademark) ST-UP)
60 parts by weight water 85 parts by weight 0.6 parts by weight sodium carboxymethylcellulose (Nippon Paper Industries, Sunrose MAC500LC) 100 parts by weight 40 parts by weight acrylic latex (manufactured by JSR, TRD202A) Then, a coating liquid was prepared, impregnated into a base material, and dried at 100 ° C. to prepare a separator having a thickness of 20 μm.

(Comparative Example 1)
A coating liquid was prepared using spherical silica (manufactured by Nissan Chemical Industries, Snowtex (registered trademark) ST-ZL) instead of the irregular-shaped silica used in the Examples, and the substrate was impregnated. A separator was produced.

[Measurement of air permeability and average pore diameter]
The air permeability of the obtained separator was measured with a Gurley densometer manufactured by Toyo Seiki. Also, the average pore size was measured by Porous Materials Inc. Measured with a Capillary Flow Porometer CEP-1500A. The results are given in Table 1.

[Evaluation of battery characteristics]
On the aluminum foil, 200 g / m ^{2 of} lithium manganate, acetylene black, and polyvinylidene fluoride were applied at a mass ratio of 100/5/3, the solvent was dried, and further pressed to produce a positive electrode. On the other hand, spherical artificial graphite, acetylene black, and polyvinylidene fluoride were coated at a mass ratio of 85/15/5 on a copper foil at a rate of 100 g / m ² , dried and pressed to prepare a negative electrode.

  A separator was sandwiched between the obtained electrodes and heated at 95 ° C. under reduced pressure of 0.01 MPa or less for 12 hours, and then an electrolyte for a lithium secondary battery manufactured by Ube Industries (trade name: Purelite, solvent: EC / DEC / DME = 1/1/1 (volume ratio), supporting electrolyte: lithium hexafluorophosphate 1 mol / l) was dropped, and sealed in an aluminum foil laminate film by reducing the pressure, and the lithium secondary battery was Produced. Next, the produced lithium secondary battery was charged to 4.2 V at 0.2 C, and then discharged at 0.2 C. At this time, the ratio of the discharge capacity initially performed under the condition of 0.2 C to the charge capacity was measured. Further, the internal resistance was measured with the voltage drop value taken as the voltage 30 minutes after the start of discharge under the condition of 0.2 C (discharge time of 300 minutes). The results are given in Table 1. Next, the battery was left to stand at 90 ° C. for 24 hours, and then charged and discharged under the same conditions of 0.2 C, and the ratio between the discharge capacity and the charge capacity was measured.

  From comparison between Example 1 and Comparative Example 1, it is clear that in Example 1, the air permeability is increased and the internal resistance is decreased. In addition, since no deterioration in characteristics due to heating was observed, an excellent separator could be obtained.

  The battery separator of the present invention can be used as a capacitor separator in addition to a lithium secondary battery separator.

Claims (1)

  A battery separator in which a porous ceramic layer containing ceramic fine particles and a support are combined, wherein the ceramic fine particles are deformed silica.